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More at http://christophe.varoqui.free.fr/faq.html
See also http://christophe.varoqui.free.fr/usage.html
1. How to set up System-on-multipath ?
======================================
prerequisite : udev and multipath-tools installed
here are the steps on a Debian SID system :
* add dm-mpath and dm-multipath to /etc/mkinitrd/modules
* copy $tools_dir/multipath/0[12]_* to /etc/mkinitrd/scripts
* define a friendly alias for your multipathed root disk
(in /etc/multipath.conf). Example : "system"
* enable busybox in /etc/mkinitrd/mkinitrd.conf and set ROOT
to any valid block-device (but not a /dev/dm-* one, due to
an mkintrd misbelief that all dm-* are managed by LVM2)
* run mkinitrd
* in /boot/grub/menu.lst, define the root= kernel parameter
Example : root=/dev/system1
* modify /etc/fstab to reference /dev/system* partitions
At reboot, you should see some info like :
path /dev/sda : multipath system
...
gpt: 0 slices
dos: 5 slices
reduced size of partition #2 to 63
Added system1 : 0 70685937 linear /dev/system 63
Added system2 : 0 63 linear /dev/system 7068600
Added system5 : 0 995967 linear /dev/system 70686063
...
2. How does it compare to commercial product XXX ?
==================================================
Here are a few distinctive features :
* you can mix HBA models, even different vendors, different speed ...
* you can mix storage controllers on your SAN, and access them all, applying
different path grouping policy
* completely event-driven model : no administration burden if you accept the
default behaviours
* extensible : you can plug your own policies if the available ones don't fill
your needs
* supports root FS on multipathed SAN
* free, open-source software
3. LVM2 doesn't see my multipathed devices as PV, what's up ?
=============================================================
By default, lvm2 does not consider device-mapper block devices (such as a
dm-crypt device) for use as physical volumes.
In order to use a dm-crypt device as an lvm2 pv, add this line to the
devices block in /etc/lvm/lvm.conf:
types = [ "device-mapper", 16 ]
If /etc/lvm/lvm.conf does not exist, you can create one based on your
current/default config like so:
lvm dumpconfig > /etc/lvm/lvm.conf
(tip from Christophe Saout)
4. I see a lot of "io_setup failed" message using the directio checker
======================================================================
The directio path checker makes use of the asynchronous I/O API (aio) provided
by modern Linux systems. Asynchronous I/O allows an application to submit I/O
requests asynchronously and be notified later of their completion status. To
allow this, we must allocate an asynchronous I/O context (an object of type
aio_context_t) and this task is handled by the io_setup system call.
A system wide limit on the number of AIO contexts that may be active
simultaneously is imposed via the aio-max-nr sysctl parameter.
Once this limit has been reached further calls to io_setup will fail with the
error number EAGAIN leading to the "io_setup failed" messages seen for e.g. when
running "multipath -ll".
To avoid this problem the number of available aio contexts should be increased
by setting the aio-max-nr parameter. This can be set on a one-time basis via the
/proc file system, for e.g.:
# echo 131072 > /proc/sys/fs/aio-max-nr
Doubles the number of available contexts from the default value of 65536.
To make this setting persistent a line may be added to /etc/sysctl.conf:
fs.aio-max-nr = 131072
Consult appropriate application and operating system tuning recommendations for
guidance on appropriate values for this parameter.
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